The present invention relates in general to electrically conductive coating compositions and more specifically to tunable conductivity coating compositions. These compositions may be employed in many applications including, for example, electrophotographic image development systems.
Electrophotographic image development systems such as liquid image development systems, scavengeless development systems and hybrid scavengeless development systems are well known in the art of electrophotography. Scavengeless development systems do not scavenge or interact with a previously toned image and thereby do not negatively affect image quality and are important in trilevel and highlight color xerography, reference for example, U.S. Pat. No. 4,078,929.
Two-phase conductive compositions are also known and contain, for example, dispersions of conductive particles, such as carbon black or graphite, in an insulating polymer matrix, for example, dielectric binders such as a phenolic resin or fluoropolymer. The conductive pigment loading concentration of two-phase conductive compositions are near the percolation threshold concentration. Conductive particle concentration levels at or near the percolation limit allow for conductive particle contact, resulting in a burst of conductivity, reference for example, U.S. Pat. No. 4,505,573, to Brewington et al. The dielectric constant of conductive coatings and overcoatings typically can be from about 3 to about 5, and preferably about 3. The desired conductivity is achieved by controlling the loading of the conductive particles. However, the low conductivity values required for electrophotographic image development systems and the large, intrinsic electrical conductivity of carbon black make it extremely difficult to achieve predictable and reproducible conductivity values. Very small changes in the loading of conductive particles near the percolation threshold can cause dramatic changes in the conductivity of a coating. Furthermore, even at a constant weight loading, differences in particle size and shape can cause wide variations in conductivity. Moreover, the percolation threshold approach to obtaining conductive coatings requires relatively high concentrations of conductive particles. At these concentrations, the coatings typically become brittle, and the mechanical properties of the coating are controlled by carbon black content rather than by the polymer matrix.
Another approach is to molecularly dope a polymer matrix with mixtures of a neutral charge transport molecule and its radical cation or anion. "Molecular doping" refers to the relatively low amounts of dopant added, compared to the aforementioned high loading concentrations of carbon black dispersions, to increase the conductivity of a polymer matrix. The resulting molecularly doped mixture is essentially a solid solution. No chemical bonding occurs between the dopant and the charge transport molecule so as to produce a new material or alloy. The doped polymer provides stable and controlled conductivity arising from molecular doping with dopants such as oxidizing agents. In the presence of an oxidizing dopant, partially oxidized charge transport moieties in the charge-transporting polymer act as hole carrier sites, which transport positive charges or "holes" through the unoxidized charge transport molecules. For example, Mort et al., J. Electronic Materials, 9:41 (1980), disclose the possibility of chemically controlling dark conductivity by co-doping a polycarbonate with neutral and oxidized species of the same molecule, tri-p-tolylamine (TTA), that is TTA, and TTA.sup.+ respectively, where TTA.sup.+ represents a cation radical salt of TTA. Limburg et al., in U.S. Pat. No. 4,338,222, disclose an electrically conducting, three-component composition comprising: a polymer matrix; an organic hole transport compound, particularly tetraaryl biphenyl diamines, and oxidized species of the same molecule, which is the reaction product of the organic hole transport compound and an oxidizing agent capable of accepting one electron from the hole transport compound. Hays et al., in U.S. Pat. No. 5,300,339 and U.S. Pat. No. 5,448,342, disclose an overcoating comprising at least three constituents: a charge transport compound, for example, especially an aryl diamine, a polymer binder, especially a polycarbonate or a polyethercarbonate, and an oxidizing agent. Hays et al., in U.S. Pat. No. 5,386,277, further disclose an overcoating comprising two constituents: a charge transport polymer with tetraaryl biphenyldiamine units in the main chain and an oxidant.